1. Field of the Invention
The present invention relates to a damping force variable valve assembly installed in a damping force variable shock absorber, and more particularly, to a damping force variable valve assembly with a low-speed control valve capable of controlling an ultra-low flow rate and a low flow rate when a damping force variable valve operates in a soft mode, and a damping force variable shock absorber including the damping force variable valve assembly.
2. Description of the Related Art
Generally, a shock absorber is installed in means of transportation such as an automobile or the like, and improves a ride comfort by absorbing and damping a vibration or shock from a road surface on the drive.
Such a shock absorber includes a cylinder and a piston rod installed to be compressible and extendable within the cylinder. The cylinder and the piston rod are installed in a vehicle body, wheels, or axles.
A shock absorber, a damping force of which is set to be low, can improve a ride comfort by absorbing a vibration caused by uneven road surface on the drive. On the contrary, a shock absorber, a damping force of which is set to be high, can improve a steering stability by suppressing a change in a posture of a vehicle body. Therefore, in the conventional vehicles, different damping force characteristics are set to shock absorbers according to the purpose of use of the vehicles.
Meanwhile, a damping force variable shock absorber has recently been developed which is mounted with a damping force variable valve at one side of the shock absorber so as to appropriately adjust a damping force characteristic, and can appropriate adjust a damping force characteristic to improve a ride comfort or a steering stability according to a road surface, a drive status, and the like.
FIG. 1 is a cross-sectional view illustrating an example of a conventional damping force variable shock absorber. The conventional damping force variable shock absorber 10 includes a base shell 12 and an inner tube 14 which is movably installed inside the base shell 12 and in which a piston rod 24 is movably installed in a length direction. A rod guide 26 and a body valve 27 are installed in an upper portion and a lower portion of the inner tube 14 and the base shell 12, respectively. In addition, a piston valve 25 is connected to one end of the piston rod 24 within the inner tube 14, and the piston valve 25 partitions an inner space of the inner tube 14 into a rebound chamber 20 and a compression chamber 22. A top cap 28 and a base cap 29 are installed in an upper portion and a lower portion of the base shell 12, respectively.
A reservoir chamber 30 is formed between the inner tube 14 and the base shell 12 to compensate for a change in a volume of the inner tube 14 according to a reciprocating motion of the piston rod 24. A flow of a working fluid between the reservoir chamber 30 and the compression chamber 22 is controlled by the body valve 27.
In addition, a separator tube 16 is installed inside the base shell 12. Due to the separator tube 16, the inside of the base shell 12 is partitioned into a high pressure chamber PH connected to the rebound chamber 20, and a low pressure chamber PL serving as the reservoir chamber 30.
The high pressure chamber PH is connected to the rebound chamber 20 through an inner hole 14a of the inner tube 14. On the other hand, the low pressure chamber PL is connected to the compression chamber 22 through a lower passage 32, which is formed between a body portion of the body valve 27 and the base shell 12 (or the base cap 29), and a passage formed in the body valve 27.
Meanwhile, the conventional shock absorber 10 includes a damping force variable valve assembly 40 mounted at one side of the base shell 12 so as to vary a damping force.
The damping force variable valve assembly 40 is provided with oil passages communicating with the high pressure chamber PH and the low pressure chamber PL connected to the base shell 12 and the separator tube 16, respectively. In addition, the damping force variable valve assembly 40 includes a spool 44 installed to be moved by a driving of a plunger 42. An inner passage communicating with the high pressure chamber PH and the low pressure chamber PL is varied by movement of the spool 44, and a damping force of the shock absorber is varied accordingly. The plunger 42 is configured to move in a horizontal direction, when viewed in FIG. 1, due to a magnetic force generated when an electric current flows through a solenoid.
In the conventional damping force variable valve assembly, for example, when the plunger 42 moves left, the spool 44 closes the passage to generate a high damping force (hard mode). On the contrary, when the plunger 42 moves right, the spool 44 opens the passage to generate a low damping force (soft mode).
In the technical field to which the invention pertains, many efforts have been made to provide a shock absorber having a good damping force variable characteristic by improving the performance of a damping force variable valve assembly. For example, Korean Patent Application Publication Nos. 10-2010-0023074 and 10-2010-0007187 disclose techniques of damping force variable valve assemblies recently developed for a shock absorber.
However, in spite of many efforts to improve the performance of the damping force variable valve assembly, the conventional shock absorber has a poor damping force characteristic in a section where a moving speed of a working fluid is low.
In the damping force variable valve assembly 40, when a moving speed of a working fluid is high, that is, when a flow rate is high, a resistance is determined by a sagging amount of main disk valves stacked in a valve assembly.
However, when an input (that is, shock) from a road surface is small and thus a moving speed of a working fluid is low or ultra-low, a flow rate of a working fluid flowing in a shock absorber is so extremely low that a resistance by a fixed orifice is not formed. Therefore, a damping force is not generated in a low-speed section, and a small vibration transferred from a road surface is not reduced.